In the oil and gas industry, reservoir modeling involves the construction of a computer model of a petroleum reservoir for the purpose of improving estimation of reserves and making decisions regarding the development of the field. For example, geological models may be created to provide a static description of the reservoir prior to production. In contrast, reservoir simulation models may be created to simulate the flow of fluids within the reservoir over its production lifetime.
With reservoir simulation models, the modeling of fractures within a reservoir can present a challenge, requiring a thorough understanding of matrix flow characteristics, fracture network connectivity, and fracture-matrix interaction. Fractures can be described as open cracks or voids within the formation and can either be naturally occurring or artificially generated from a wellbore. The correct modeling of the fractures is important as the properties of fractures such as spatial distribution, aperture, length, height, conductivity, and connectivity significantly affect the flow of reservoir fluids to the well bore.
Mesh generation techniques are commonly used in reservoir modeling. Two common mesh generation techniques, for three-dimensional reservoir simulation, are structured-based meshing and extrusion based meshing. In the most typical structured techniques, hexahedra are connected in a logical i-j-k space with each interior mesh node being adjacent to 8 hexahedra. Several extensions and generalizations exist, including Local Grid Refinement where local regions of an original grid are replaced with finer grids. This meshing technique works well for a variety of reservoir configurations, but can become time-consuming and prohibitively burdensome for users when dealing with general reservoir geometries, like arbitrary three-dimensional fracture surface. Because of the inherent 2.5 dimensional nature of extrusion techniques, similar limitations apply to these techniques. Alternatively, fully-unstructured meshing techniques do exist, including tetrahedralization and polyhedral meshing schemes. The increased complexity of these fully-unstructured techniques often leads to lower robustness, when compared to the structured method, especially, in the presence of imperfect geometry input (“dirty geometry”).